There are a variety of analytical testing procedures and experiments which must be undertaken in low temperature environments, typically in the temperature range of between about 3 degrees Kelvin to about 200 degrees Kelvin (3.degree. K to about 200.degree. K). Conventional sample cooling and testing apparatus designs utilize liquid nitrogen, liquid helium, or both to obtain the required low temperatures. Implementation and application of these mildly hazardous materials is generally costly, labor intensive, and inconvenient as a result of the relatively high cost of the materials themselves, the frequent transfers from storage containers to experimental apparatus, and the equipment necessary to confine and handle such materials safely and efficiently. Analyses and experiments requiring substantial time periods at reduced temperatures also pose significant hurdles in maintaining the experimental setups at the necessary low temperatures.
Recently, however, advancements in technology have enabled the use of closed-cycle helium refrigerators in place of the low temperature liquids, as, for example, described in the article entitled The Use of a Helium Refrigerator for Mossbauer Studies, by Y. Chow, et al. (Vol. 66, Nuclear Instruments and Methods, 1968). As mentioned by Chow et al., in order to obtain appreciable Mossbauer effect, it is often necessary for the gamma-ray emitter and/or absorber (sample) to be cooled to cryogenic temperatures. While the helium refrigerator has been found to be an efficient and cost effective alternative to the use of expensive cryogens, practical applications of these devices are plagued by vibrations from the refrigerator itself, including vibrations associated with compression/decompression of the helium gas, as well as the motion of pistons in their cylinders. The result of these mechanical vibrations has severely limited the application and usefulness of these helium refrigerator devices in the more vibration-sensitive techniques such as Mossbauer spectroscopy and the like.
A number of approaches directed to obtaining relatively vibration-free sample cooling have been attempted in the industry. Examples include: attachment of the sample holder to the refrigerator cooling mechanism through flexible copper braids to provide a non-rigid thermal conduction path, as seen in U.S. Pat. No. 4,854,131 (Sakitani et al.); attaching a gas-filled envelope through a flexible bellows-like device for both thermal conduction and mechanical vibration isolation, as shown in U.S. Pat. No. 3,894,403; and providing gas by-pass arrangements to reduce inlet and outlet pressure differentials. While all of these approaches have been successful to some degree, none have provided a complete solution to the problem of isolating refrigerator vibrations in applications for sample cooling. For example, for the copper braid-type arrangement to provide an effective thermal conduction path, it must possess a sufficiently large cross-section, which inherently results in a compromise of the vibration isolation benefit, as some vibrations are always transmitted through the flexible braids. Some experiments have been conducted by turning the refrigerator compressor off while acquiring data. Long term low temperature maintenance is clearly a problem in such cases. Similarly, the use of by-pass lines and the like reduces, but does not eliminate, the vibrations which are inherent in these refrigerators. Moreover, by-passing arrangements do not eliminate the mechanical vibrations associated with the moving pistons of the refrigerator expander.
Taking a closer look at the vibration-free refrigeration transfer device of U.S. Pat. No. 3,894,403, there is described a mechanical device which decouples vibrations of the refrigerator from the sample by utilizing a helium exchange gas provided in an envelope that surrounds the refrigerator itself. While this gas envelope provides a larger area for thermal conduction and effectively isolates the mechanical vibrations, the arrangement shown and described in this patent fails to adequately address the arrangement for mounting the refrigeration system relative to the measurement system. The '403 patent merely suggests that a rigid support system be provided in order to derive the maximum benefit from the vibration isolation aspects of the invention, proposing a rail-like structure for rigidly attaching transducers and the gamma ray optics used in Mossbauer spectroscopy to a tabletop. No structure for vibration isolation or damping to prevent transmission of vibrations from the surrounding environment to the experimental equipment is disclosed in this patent document, and there is no true isolation provided between the refrigerator expander unit and the sample mounting structure. As a result, even the improved refrigeration transfer arrangement of this patent could not provide the essentially vibration-free system required for reliable use in conjunction with Mossbauer and other vibration-sensitive studies.
Moreover, a number of attempts have been made to provide adequate mounting of closed-cycle refrigerators for the purpose of eliminating vibration. None have been thoroughly described in the literature as successfully utilizing the refrigeration transfer arrangement described in the '403 patent.
U.S. Pat. No. 4,161,747, entitled SHOCK ISOLATOR FOR OPERATING A DIODE LASER ON A CLOSED CYCLE REFRIGERATOR, describes a mount which is rigidly affixed to a bench or table and upon which the laser diode is mounted. The vacuum shroud is attached to the bench, and the expander portion of the refrigerator is attached to the laser diode mount through a plurality of thermally conductive, flexible straps. The vibrations of the expander unit are isolated from the bench and mount assembly by connecting the vacuum shroud and its radiation shield to the expander by means of flexible vacuum hoses. While this mounting system is relatively compact, this arrangement suffers critical residual vibrations with amplitudes of nearly 2300 nm or more because the conductive straps provide a path through which vibration can be transmitted to the laser diode mount, and because the vacuum hoses are incapable of totally absorbing shock and vibration along their central axis. As a result, vibration is transmitted to the vacuum shroud, the bench and the diode mount.
U.S. Pat. No. 4,384,819, entitled VIBRATION ISOLATION AND PRESSURE COMPENSATION APPARATUS FOR SENSITIVE INSTRUMENTATION, describes a mounting arrangement for closed-cycle refrigerator which allegedly isolates the refrigerators from vibration-sensitive instruments while permitting thermal contact therebetween. The patent describes the combination of flexible, thermally conductive straps to couple the sensitive instruments to the cold end of the refrigerator, a housing assembly to be secured to an instrument platform, a reaction bracket supporting the refrigerator expander unit and absorbing shock and vibration therefrom, a cryocooler, and a flexible bellows arrangement to physically attach the cryocooler to the instrument chamber and reaction bracket. As set forth in this patent, however, the resulting line width broadening of CCl.sub.2 is increased by 2 MHz, or about 5.4%. An equivalent line broadening measured by Mossbauer spectroscopy would yield a residual vibration amplitude of nearly a full micron of motion.
Moreover, the apparatus of the '819 patent is contemplated for specialized applications such as airborne instrumentation and the like, and is relatively limited in applications outside of that field. In any case, while this system appears to provide some improvement in accuracy of experimental results, the residual vibration levels of the system continue to remain above acceptable levels for applications such as Mossbauer spectroscopy.
The very low temperature refrigerator shown and described in U.S. Pat. No. 4,854,131 includes a mounting system for rigidly suspending the refrigerator in a cryostat, while providing for vibration isolation therebetween. This structural setup resembles the refrigeration transfer interface of U.S. Pat. No. 3,894,403 described above, but employs a suspension system comprising an array of wires which are connected tangentially at various points along the cold end of the refrigerator. The cold end of the refrigerator, is, in turn, connected to the sample by way of a flexible, thermally conductive strap. While this mounting arrangement provides a relatively compact structure, the residual vibration amplitude at low frequency is nearly 1000 nm, and is substantially above acceptable levels for some of the more vibration-sensitive procedures.
U.S. Pat. Nos. 4,363,217; 4,833,899; 4,835,972; and 4,862,697 pertain to structures contemplated for vibration damping and isolation of cryopumps. In particular, these patents illustrate the suspension of the refrigerator expander head having a weight attached to dampen vibratory motions; and, alternatively, hanging the refrigerator expander head from a self-supporting bellows configuration. While the weight of the first arrangement does not tend to move precisely out of phase with the refrigerator, the vibratory amplitude is significantly damped to permit proper operation of some vibration-sensitive equipment, such as electron microscopes. While such active vibration damping arrangements have some meaningful usage with respect to cryopumps and relatively less sensitive equipment and procedures, they do not provide the high level of vibration isolation required for sample cooling and low temperature experiments which nominally require freedom from vibration.
Consequently, the structures and methods heretofore available for mounting of closed-cycle helium refrigerators for sample cooling and low temperature spectroscopy have failed to provide the required substantially vibration-free environments necessary for optimum performance and resolution. In this regard, there has not been available an adequate solution to the problem of refrigerator vibrations in applications for sample cooling, as residual vibration amplitudes persist above acceptable levels. Similarly, there has not been available a mounting system which provides true isolation between the refrigerator expander unit and the sample mount, and vibrations present in the ambient environment due to the operation of the refrigerator compressor have not been prevented from being transmitted directly to the experimental apparatus. Available methods simply did not hold the sample mount stationary relative to measurement devices while simultaneously providing for the low temperatures and flexibility for adaptations required to undertake a variety of sample cooling and spectroscopy operations.